1. Field of the Invention
The present invention relates to a method of and an apparatus for bending two overlapping glass plates simultaneously in a heating furnace. Particularly, the present invention relates to a method of and an apparatus for bending two overlapping glass plates which is suitable for deep-bending a side portion of the overlapping glass plates.
2. Discussion of Background
A laminated glass is formed by two glass plates and an intermediate plastic film such as polyvinyl butyral sandwiched therebetween, and it is widely used for a front glass as a wind-shielding window for an automobile from the standpoint of safety.
Because of requirements concerning the design of automobiles, a curved laminated glass is required. Accordingly, it is necessary to bend flat glass plates. In this case, when the glass plates are separately bent, a delicate difference in shape appears between the glass plates to be laminated. Accordingly, when they are laminated with the intermediate layer interposed therebetween, there occurs disadvantages that a complete joint between the two glass plates is not obtainable, or air bubbles are resulted at the surfaces of bonding. Accordingly, a method of bending simultaneously the two overlapping glass plates has been used for manufacturing the laminated glass. As a conventional method of bending glass plates for a laminated glass, there is known such a method that a bending mold having a bend-shaping surface corresponding to a curved surface of the laminated glass is prepared; two glass plates are placed on the bending mold in an overlapping state; the bending mold is transferred to a heating furnace; and the glass plates are heated to a temperature capable of softening glass so that the glass plates are bent by their own deadweight so as to correspond to the bend-shaping surface of the bending mold when the plates are softened (Japanese Examined Patent Publication No. 49-10332).
In such method, when it is necessary to bend deeply a side portion of the laminated glass, the bending mold is constituted by a fixed split mold in a ring form which has a bend-shaping surface corresponding to a middle curved portion of the laminated glass and a movable split mold (or movable split molds) in a ring form which has a bend-shaping surface corresponding to a portion to be deeply bent in a side portion of the laminated glass, and which is placed at a side (or both sides) of the fixed split mold so that it can move to the fixed mold so as to be in alignment with the fixed mold by its own deadweight. Two glass plates in a flat form are placed on the bending mold with the movable split mold (molds) being developed; the bending mold is transferred to a heating furnace along with the glass plates; the glass plates are heated to cause the softening of the glass plates; and the movable mold (molds) is moved to come in alignment with the fixed mold while the glass plates are softened, whereby a side portion of the glass plates is deeply bent by their deadweight by means of the movable mold (molds) (Japanese Examined Patent Publication No. 48-1210).
However, the conventional methods of bending the glass plates have the following disadvantages.
When the side portion of the glass plates are to be deeply bent, the side portion of the glass plates is forcibly bent by means of the movable mold which is movable by its deadweight. In such bending mold, when a degree of deep-bending of the side portion of the glass plates is increased, shaping the deep-bent portion of the glass plates is insufficient because it is difficult for the glass plates to follow a desired radius of curvature at the curved portion except for the peripheral portion of the deeply-bent glass plates, although the peripheral portion of the deep-bent glass plates can be shaped in compliance with the shape of the movable mold. Further, a portion to be deeply bent is locally heated so that it can be easily bent. However, such method is still insufficient.
In order to overcome the above-mentioned problem, it can be considered to use a pressing method by which glass plates are shaped between a pair of curved molds to thereby form a correctly curved shape of the glass plates. In such pressing method, it is necessary to hold two glass plates at a correct position in order to shape the two glass plates simultaneously to form a laminated glass. However, it is difficult to hold the overlapping glass plates at a correct position by using a known holding technique such as hanging or sucking. As a result, the pressing method can not be practically used.
As another solution, there is proposed such a method in which the two glass plates have a size greater than the desired final product of laminated glass; the two glass plates are put one on another on a bending mold, and are subjected to bending operations simultaneously to obtain a desired curved shape, and then the circumferential portions of the glass plates are cut; thus a laminated glass having a desired dimensions is obtainable. This method is advantageous in that the bending operations of the side portions of the glass plates can be easy. However, an additional cutting step is needed after the bending operations, and it requires much labor and cost.